1. Field of the Invention
The present invention relates to a digital exchange such as a digital PBX (Private Branch exchange), and more particularly to control of a highway forming a communications path of a digital PBX.
2. Description of the Prior Art
In a digital exchange such as a digital PBX, a communications path for transferring voice and data signals is generally called a highway.
FIG. 1 shows a conventional digital PBX of a distributed control type. The digital exchange shown in FIG. 1 includes a time division switch SW, a plurality of line trunk (accommodating) units LTU0 through LTUn, and a control unit CU. Each of the line trunk units LTU0-LTUn includes a plurality of line trunk packages LT0-LT15, and is accommodated in a frame of the digital PBX. The line trunk packages LT0-LT15 control line processors LPR0-LPR15, respectively. The line trunk packages LT0-LT15 accommodate lines connected to telephone sets or public networks. Each of the line trunk packages LT0-LT15 accommodates 16 lines. The line trunk units LTU0-LTUn are connectable to the time division switch SW via the highways HW.
Data transferred via the highway HW is formed as shown in FIG. 2, which shows the format of data on the highway HW connecting the time division switch SW and one line trunk unit LTU to each other. As shown in FIG. 2, the data includes a plurality of frame units F0-F32. One frame includes 32 time slots TS0-TS31. One time slot consists of eight bits B7-B0.
The bit rate of a data transfer via a conventional digital PBX using a sampling frequency of 8 kHz and eight quantization bits is 64 kbps (=8 kHz.times.8 bits). When data (equal to eight bits) per subscriber is represented by one time slot, one frame corresponds to a synchronizing frequency of 8 kHz (=125 .mu.s), and one frame consists of 32 time slots, then one time slot is as follows: EQU 1TS=125 .mu.s (1F)/32.apprxeq.3.9 .mu.s.
Each of the eight bits B7-B0 in one time slot is as follows: EQU 1 bit=3.9 .mu.s/8.apprxeq.488 ns=2.048 MHz.
Normally, the data transfer direction in which data is transferred from the line trunk package LT to the time division switch SW via the highway HW is called the up-transfer direction, and the data transfer direction in which data is transferred from the time division switch SW to the line trunk package LT via the highway HW is called the down-transfer direction. Each of the line trunk units LTU0-LTU15 (n=15) simultaneously handles a plurality of highways HW.
FIG. 3 shows the structure of the line trunk unit LTU0. In FIG. 3, the line trunk packages LT0-LT15 shown in FIG. 1 are omitted. The line trunk unit LTU0 includes an interface HWINF having multiplexer/demultiplexer units (MPX/DMPX) 10 and 12. Symbols HW0-HW7 indicate highways for transferring speech data, and symbol HWC indicates a highway for transferring communications data. A set of highways HW0-HW7 and HWC is provided for the up-transfer direction, and another set of highways HWO-HW7 and HWC is provided for the down-transfer direction. The communications data includes a variety of control data used for the distributed control of the digital PBX.
The interface HWINF establishes an interface between the time division switch SW and the line trunk unit LTU0. The MPX/DMPX unit 10 performs a multiplexing/demultiplexing process for highways. The MPX/DMPX unit 12 performs a multiplexing/demultiplexing process for the highways HW0 and HWC. Dots shown in FIG. 3 denote points (contacts) used in slots for accommodating the line trunk packages LT0-LT15, these points being formed in the frame of the digital PBX. Slender blocks including points denote slots (LT) for accommodating the packages, these slots being provided in the frame of the digital PBX. The structure shown in FIG. 3 has 16 slots 0-15. For example, line trunk packages to be inserted into slots 0 and 1 share the highway HW7, that is, 32 time slots formed on the highway HW7. The communications data highway HWC utilizes a part of the speech data highway HW0, and is shared by the 16 slots.
Returning now to FIG. 1, the time division switch SW includes a multiplexer (MPX) 21, a speech path memory (SPM) 22, and a demultiplexer (DMPX) 23. The multiplexer 21 multiplexes data transferred via the highways HW from the line trunk units LTU0-LTU15 and communications data from a control unit CU, which will be described later, and output multiplexed data to the speech path memory 22. Under the control of the control unit CU, the speech path memory 22 performs a switching operation in which the time slots are switched. The demultiplexer 23 performs a distributing process for data read from the speech path memory 22, and transmits the read data to the line trunk units LTU0-LTU15 via the highways HW. Further, the demultiplexer 23 outputs the communications data to the control unit CU.
The control unit CU controls switching of the time division switch SW and controls the transmitting and receiving operations on communications data (control data) transferred between the control unit CU and the line processors LPR0-LPR15. The control unit CU includes a call processor CPR, a communications data buffer memory (signaling memory) SM, a speech path memory controller CTL, and a processor bus PB connecting these structural parts. The call processor CPR performs control for communicating with the line trunk units LTU0-LTU15 and a maintenance operation thereon. The speech path memory controller CTL controls the speech path memory 22. The communications data buffer memory SM is used to hold the communications data.
More particularly, the communications data buffer memory SM includes a send buffer memory SSM and a receive buffer memory RSM. The send buffer memory SSM holds communications data transferred in the down-transfer direction (in which the communications data is transferred from the time division switch SW to the line trunk units LTU). The receive buffer memory RSM holds communications data transferred in the up-transfer direction (in which the communications data is transferred from the line trunk units LTU to the time division switch SW).
FIG. 4 is a diagram of the structures of the send buffer memory SSM and the receive buffer memory RSM. Each of the send buffer memory SSM and the receive buffer memory RSM has buffer areas respectively assigned to the line trunk units LTU0-LTU15 (n=15). Each of the buffer areas has buffer areas respectively assigned to the line processors LPR0-LPR15. When one-byte communications data is used per one line processor LPR, areas equal to 16 bytes in total are permanently allotted in advance in each of the send buffer memory SSM and the receive buffer memory RSM.
For example, the communications data from a line processor LPR of the line trunk unit LTU0 is transferred via the communications data highways HWC which is a part of the aforementioned speech data highway HW0, and is input to the multiplexer 21 in which the input communications data is multiplexed with other data. Under the control of the speech path memory controller CTL, the speech path memory 22 switches the time slots, and writes the above-mentioned communications data into the corresponding area in the receive buffer memory RSM. The communications data to a line processor LPR of the line trunk unit LTU0 is written into the corresponding buffer area in the send buffer memory SSM under the control of the call processor CPR, and is then multiplexed with other data by the multiplexer 21. Then, the communications data is subjected to the process for switching time slots in the speech path memory 22, and is demultiplexed by the demultiplexer 23. Then, the demultiplexed communications data is transferred to the call processor CPRF.
However, the above-mentioned conventional communications system has a disadvantage caused by the structure in which one line trunk unit LTU handles a fixed number of highways. For example, in the structure shown in FIG. 15, one line trunk unit LTU can handle eight highways. This is due to the fact that the system structure needs a predetermined number of line processors LPR which can be accommodated in one line trunk unit LTU.
As shown in FIG. 3, when the line trunk unit LTU0 is designed to handle eight highways HW at most, 16 slots capable of accommodating 16 line processors LPR are provided in the frame of the digital PBX. Hence, although all of the 16 slots may be used in the future, the system structure is not efficiently utilized if a smaller number of slots are initially used at present. In this case, the space for placing the digital PBX is not efficiently used.
The above problems are particularly serious in the following case.
Generally, in the communications system shown in FIG. 1, the lines connecting the line trunk units LTU to the terminals and the public networks are metallic lines, which can be allowed to extend 1 km at most without any repeater. With the above in mind, use of a remote line control system has been proposed.
FIG. 5 is a block diagram of a conventional digital PBX using a remote line control system. In FIG. 5, parts that are the same as those shown in FIG. 1 are given the same reference numbers as previously. As shown in FIG. 5, a remote unit RU is substituted for the line trunk unit LTU0, and a remote interface unit RIF for connecting the time division switch SW and the remote unit RU via a transmission line is provided at the side of the digital PBX. The remote unit RU and the remote interface unit RIF are control devices for extending the distance of highways (transmission line), and allows the transmission line to extend to a length allowed by the transmission line interface. In practice, it is possible to form a transmission line tens of kilometers in length.
The remote unit RU includes a remote unit controller RUCTL having line trunk packages LT0-LT15, which respectively establish interfaces with the transmission line. As has been described previously, the number of slots (highways) in the line trunk unit LTU is fixed, and a predetermined number of slots (16 slots in the example shown in FIG. 3) is mounted irrespective of how many slots are actually in use. Similarly, the number of slots (highways) in the remote unit RU placed in a remote place is fixed. Hence, a predetermined number of slots (16 slots corresponding to eight highways in the example shown in FIG. 3) is provided in the remote unit RU irrespective of how many slots are in use.
Even if the number of slots provided in the remote unit RU is reduced, the number of highways allotted to the remote unit RU is not changed. Hence, there are highways which are not used and the system cannot operate effectively.